1. Field of the Invention
This invention relates to a drive method for a brushless motor and a drive control apparatus therefor. More particularly, it relates to a drive method for a brushless motor and a drive control apparatus therefor as are well suited for application to a hydraulic type power steering control system employing the brushless motor.
This application is an application relevant to U.S. patent application Ser. No. 11/311,313 filed on Dec. 20, 2005 (corresponding Japanese Patent Application: Application No. JP2005-145429 filed on May 18, 2005) by the same inventors as in this application.
2. Description of the Related Art
In recent years, power steering control systems each of which assists the driver of a vehicle in handle manipulation have been often used for the steerability and stability of the driver of the vehicle. The power steering control systems include an electric type and a hydraulic type. As compared with the hydraulic type, the electric type has such merits that assist characteristics which support the driver can be freely set at the time of design, that the versatility of the control system is high, and that the fuel consumption of the vehicle is enhanced. On the other hand, however, the electric type has problems in the steerability and an environmental immunity including heat or noise.
Especially in a large-sized vehicle, therefore, the hydraulic type power steering control system is often adopted. A brushless motor is adopted for the hydraulic type power steering control system. In the hydraulic type power steering control system employing the brushless motor, a hydraulic pump is driven by the brushless motor, and a predetermined assist force as designed can be realized by a controlled hydraulic pressure in accordance with a vehicle running condition such as a vehicle speed or a steering angle rate.
In general, the hydraulic type power steering control system adopting the brushless motor stores the relationship between the vehicle speed or the steering angle rate and the designed revolution number of the brushless motor, in the microcomputer (hereinbelow, termed “memory”) of the control system beforehand. Besides, the revolution number of the brushless motor is detected by a sensor for detecting the motor rotation position of the brushless motor, the deviation between the actual revolution number detected and the target revolution number stored in the memory is calculated, and the revolution number of the brushless motor is controlled so that the value of the deviation may become, at most, a predetermined value as designed, whereby the assist force required by the driver is obtained. Such a control technique is partly disclosed in, for example, JP-A-10-70894.
Also, JP-A-7-337067 states the fact that the conduction phase angle of a motor drive circuit is controlled on the basis of the revolution number and current of the brushless motor.
With the prior-art control technique disclosed in JP-A-10-70894, the drive timing (hereinbelow, termed “conduction phase angle”) of the pulse width modulation (hereinbelow, abbreviated to “PWM”) signal of a drive circuit for driving the brushless motor is set at a fixed angle. Accordingly, no special problem occurs in the brushless motor in the state where it revolves at a constant speed. However, in a case where a variable speed is required, there is such a problem that a region in which an efficiency lowers or a consumed current increases appears.
Besides, it is disclosed in JP-A-7-337067 that the conduction phase angle of the drive circuit for the brushless motor is controlled on the basis of the revolution number and current of the brushless motor. Thus, advantages such as the enhancement of an efficiency and the decrease of a consumed current are noted in a steady running state at a predetermined revolution number. In order to perform a control in a wider range of revolution numbers, however, the responsibility of the conduction phase angle of PWM needs to be heightened.